Meg Rosoff: “Be as adventurous as you can! Don't aim for the middle!” Photograph: Zoe Norfolk

Meet Mila, the 12-year-old protagonist of Meg's Rosoff's seventh novel, Picture Me Gone. She's a girl with a talent for sniffing out truths:

"Like my namesake, Mila the dog, I have a keen awareness of where I am and what I'm doing at all times. I am not given to dreaminess, have something of a terrier's determination. If there is something to notice, I will notice it first."

Mila and her father, Gil, are poised to visit Gil's old friend Matthew in New York when they receive the news that he has run away. They go anyway, arriving at Matthew's home to discover his angry, embittered wife Suzanne, joyful baby Gabriel and the sad, lonely and unfathomable Honey, Mathew's dog. Why did he leave her behind? And then there is Owen, Matthew and Suzanne's first child, who died in a car crash.

At the heart of the book are a series of questions: Why has Matthew left? Where has he gone? Is he coming back?

Rosoff's debut novel, How I Live Now won the 2004 Guardian children's fiction prize and the Branford Boase award and established her as a writer of emotional intensity. The story of Daisy and her search for belonging against the backdrop of a sudden and unpredictable war in England has since been made into a film, out on 4 October in the UK. Her second novel, Just in Case, won the Carnegie medal in 2007. This latest story marks a return to the emotional richness of How I Live Now.

"Why Matthew left, I did not know," Rosoff says. "Writing is holding your nerve, waiting for the character to tell you...what? Mila; she is me. We were inseparable while I was writing [the book]. Her questions are my questions. The tension in the story is my tension."

Rosoff, who in person is forthright and disarmingly open, says that Mila was first written on her blog [http://www.megrosoff.co.uk]. "I was waiting for the story to arrive, not forcing it. I was waiting and panicking, waiting and panicking. My editor was bugging me and I was saying, 'It's fine, fine, fine'. I wrote a blog about Mila, even though there was no character. I knew it would shut her up for a while."

Walking her two lurchers on Hampstead Heath, Rosoff met a Bedlington terrier called Mila, found her opening line, and began writing.

"A story was there in my head. I don't know how it works. There's a weird element of magic that has to do with the quality of your subconscious mind," Rosoff says.

Understanding her subconscious workings is central to how Rosoff explains her writing and herself. "I found the world very confusing for a very long time. I couldn't see clearly, I kept misfiring. I kept falling in love with people who weren't interested in me. I was sorted out by a fantastic shrink in my late 20s, and moved to the UK," she says.

"My adolescence went on into my 40s: Who am I? What will I do with my life? Who will I love? There are periods when you stop asking those questions, they get put on hold – your first job, marriage, settling down in your 20s and 30s.

"I was a shaky hologram. How can you fall in love with a hologram? It took a long time for me to be able to see myself clearly and because of that, I've thought it all through: identity, love. I can write about people."

Rosoff refers throughout our interview to horse riding, which she took up when she was 50 (she's now 56). "It's my heart," she says. "I has made me happier than anything in the world. I'm fearless. A mixed blessing at my age, I've had five or six concussions. I'm so demented anyway, and I have such a bad memory. . ."

She does not seem at all demented, but she definitely comes across as fearless. "There is no magic bullet [to riding], it's work, work. Endless doing until it takes on an extra life. There is this concept of 'throughness' in dressage, the cycle of energy between rider and horse. The horse is the unconscious, a big powerful thing that you don't have total control over, but somehow you have to make it do your bidding."

She is as much describing her writing process as horse riding. "Be as adventurous as you can! Don't aim for the middle!"

Mila – Rosoff – is in fearless pursuit of truth in Picture Me Gone, in a way that the adults around her can find disturbing. Rosoff several times refers to her as "a prescient child"; it chimes with her description of writing as "a kind of alchemy", from which emotional truths or revelations can emerge.

At the centre of Picture Me Gone are Mila's relationships to, and understanding of, the adults in her world. There is Gil, of whom she says: "It is better for him to have company when he travels, to keep him on track," and her temporarily distant mother, Marieka. There is Matthew, the runaway, and his flaky ex-girlfriend Lynda who is still semi-dependent on her old flame. And Suzanne, who Rosoff initally paints as a bitter, negative woman but whose transformation in Mila's eyes reflects her growing recognition of the imperfections of the adult world.

"Gill and Marieka lie to Mila. The whole trip is a kind of lie," says Rosoff. "She knows there is something wrong; why take the scenic route?

"The adults have all slightly failed the children. Parents do fail children. When children realise that, it allows them to let go of that huge terrifying dependency on their parents. The process of becoming an adult is realising that your parents are fallible. I love the age of 12, because it's the last age when your parents are the centre of your life.

"The children in the book have the possibility to do it better and to think, 'I won't be miserable'."